Fuel cells are roughly classified into phosphoric acid type, alkaline type, molten carbonate type, solid oxide type, and solid polymer type, according to the kind of the electrolyte they employ. Among them, solid polymer fuel cells (PEFCs), which are capable of operating at low temperatures and have high output densities, are becoming commercially practical in such applications as automobile power sources and domestic cogeneration systems.
Meanwhile, portable appliances, such as notebook personal computers, cellular phones and PDAs, have recently been becoming more and more sophisticated, and the electric power consumed thereby tends to increase commensurately. Such portable appliances are currently powered by lithium ion secondary batteries and nickel-metal hydride secondary batteries, but manufacturers of these batteries have failed to improve energy density so as to keep up with the recent increase in power consumption. Under such circumstances, a problem of capacity shortage of power sources is a matter of concern.
As a power source that can solve this problem, PEFCs have been receiving attention. Among them, direct oxidation fuel cells (DOFCs) can generate electric energy by directly oxidizing fuel at the electrode, without the need to reform fuel that is liquid at ordinary temperature into hydrogen. DOFCs are most promising in that they need no reformer and permit an easy reduction in the size of power sources.
With respect to the fuel to be fed to DOFCs, low-molecular-weight alcohols and ethers have been examined. Among them, methanol offers a high energy efficiency and a high output. Thus, direct methanol fuel cells (DMFCs), which use methanol as the fuel, are most promising.
The anodic and cathodic reactions of a DMFC are represented by the following formulae (1) and (2), respectively. Oxygen serving as the oxidant on the cathode is typically taken in from air.CH3OH+H2O→CO2+6H++6e−  (1)3/2O2+6H++6e−→3H2O  (2)
The protons produced on the anode migrate to the cathode through an electrolyte membrane, and a perfluorosulfonic acid film, typically Nafion (Nafion is a trademark of E.I. Du Pont de Nemours & Company), has been used as the electrolyte membrane in the same manner as in PEFCs.
The polymer constituting a perfluorosulfonic acid film is commonly composed of a carbon fluoride chain (main skeleton) and branched chains each having a sulfonic acid group at the terminal thereof. The sulfonic acid groups are strongly hydrophilic, while the carbon fluoride chain is strongly hydrophobic. Thus, it is considered that the perfluorosulfonic acid electrolyte undergoes a phase separation, thereby forming water clusters that are surrounded by hydrophilic sulfonic acid groups. Further, it is considered that when the perfluorosulfonic acid electrolyte is hydrated, many protons are dissociated from the strongly acid sulfonic acid groups, thereby exhibiting excellent proton conductivity.
On each side of the electrolyte membrane is usually formed a catalyst layer, which contains a catalytic substance. In order to ensure the proton conductivity inside the catalyst layer, the catalyst layer is commonly formed by mixing a solution containing perfluorosulfonic acid, which is the same component as that of the electrolyte membrane, with a catalytic substance, applying the mixture and drying it.
Commonly used indexes that indicate the amount of sulfonic acid groups in perfluorosulfonic acid polymer are ion exchange capacity and equivalent weight (hereinafter “EW values”). The former indicates the equivalent of sulfonic acid per unit dry resin weight, being expressed in a unit of, for example, mEq/g. The latter indicates the dry polymer weight per 1 equivalent of sulfonic acid groups, being expressed in g/Eq. The values of these two indexes are, as it were, reciprocal numbers. Although either of the two can be used, the EW values are used herein.
It should be noted that “EW values” and “ion exchange capacity” are applicable not only to perfluorosulfonic acid polymer, but also other ion exchange polymers composed mainly of a hydrocarbon and copolymers including an inorganic substance.
As the EW value is higher, the ratio of sulfonic acid groups in the polymer decreases and therefore the amount of proton dissociation decreases. Hence, the proton conductivity lowers and the resistance to ionic conduction in the catalyst layer increases, thereby leading to degradation in power generation performance.
On the other hand, if the amount of sulfonic acid groups is increased, i.e., if the EW value is lowered, the water-retaining ability of the polymer is enhanced. As a result, water produced at the cathode of the fuel cell accumulates inside the electrode, thereby interfering with the supply of oxygen (oxidant). This problem is known as “flooding”.
In order to solve this problem, Japanese Laid-Open Patent Publication No. Hei 9-213350 proposes a technique associated with PEFCs that use hydrogen as the fuel. This document proposes that the ion exchange capacity of the ion exchange resin contained in the anode catalyst layer be greater than that of the ion exchange resin contained in the cathode catalyst layer. According to this proposal, the ionic conductivity of the anode is maintained at a high level while the flooding phenomenon of the cathode is prevented, so that PEFCs with a high current density and little deterioration in battery characteristics can be obtained.
Such proposal is considered to be effective for PEFCs that use hydrogen as the fuel. However, the present inventors have found that the technique disclosed in the above-mentioned document is not effective with respect to DOFCs, such as DMFCs, in which an organic fuel is directly supplied to the anode, in terms of obtaining an excellent long-term performance, because there are differences in the characteristics of deterioration of electrode performance between DOFCs and PEFCs. These differences are described below.
First, the deterioration of cathode performance due to the flooding phenomenon is described. In the same manner as PEFCs, DOFCs such as DMFCs also suffer from this problem. That is, water is produced at the cathode as represented by the formula (2), and there is an electro-osmotic drag of water by protons that move from the anode to the cathode (this water is hereinafter referred to as dragged water).
However, when PEFCs and DMFCs are compared, PEFCs can produce a dramatically higher voltage, and therefore PEFCs can operate at a larger current density than DMFCs in order to produce a higher output. In fact, while the above-mentioned document states that the initial current density of the PEFC is approximately 0.9 A/cm2, the current density of DMFCs is 0.3 A/cm2 at maximum, preferably approximately 0.2 A/cm2.
In an electrochemical reaction, by Faraday's law, the amount of products is proportional to the current generated, and the amount of electro-osmotic drag of water is proportional to the amount of proton migration, i.e., current. Therefore, both the amount of water produced at the cathode and the amount of dragged water are proportional to the current, i.e., current density. This indicates that the amount of water causing cathode flooding in a DMFC is approximately ⅓ of the amount in a PEFC.
Next, the performance deterioration upon a long-term operation is described. The present inventors have found that DOFCs such as DMFCs exhibit a performance deterioration phenomenon upon a long-term operation, but this phenomenon does not remarkably occur in PEFCs.
Electrolytes capable of conducting protons at low temperatures, such as perfluorosulfonic acid, exhibit high proton conductivity when they absorb water and a proton conductive path is formed by the water. However, upon absorption of water, these ion exchange resins usually swell depending on the amount of water absorbed until the water content reaches saturation, and their volume therefore increases. This volume increase is remarkable when an organic fuel such as methanol is used in place of water. For example, the volume change that occurs when Nafion (registered trademark), which is a representative perfluorosulfonic acid, is immersed in water is approximately 30%, while the volume change that occurs when it is immersed in methanol is as much as 130%.
That is, when an aqueous solution containing an organic fuel as the fuel is supplied to the anode, the ion exchange resin contained in the anode swells significantly, and its volume therefore increases. Such swelling is more remarkable with an increase in fuel concentration.
The present inventors have found that the deterioration of electrode performance is promoted as follows. First, an uneven fuel concentration inside the electrode leads to an uneven degree of electrolyte swelling, thereby creating a mechanical stress inside the electrode. Further, during a long-term operation of a fuel cell, for example, due to changes in operating conditions such as temperature and current, or at the time of start and stop of the operation, the fuel concentration inside the electrode changes over time, so that the electrolyte repeatedly swells and contracts. The deterioration of electrode performance is promoted by such mechanical stress, repetitive electrolyte swelling and contraction, and combination thereof.
This is described more specifically. During power generation, the fuel concentration of the catalyst layer inside the electrode is highest at the surface in contact with the gas diffusion layer, i.e., the outer surface of the electrode, while it is lowest at the surface in contact with the electrolyte membrane. This is because the fuel is consumed by oxidation reaction inside the electrode, or the fuel migrates to the cathode due to the crossover phenomenon.
Thus, when the relation between power generation and fuel supply is normal, the volume increase is greater in a region of the catalyst layer closer to the gas diffusion layer, while the volume increase is less in a region closer to the electrolyte membrane. However, once the supply-demand relationship of fuel becomes out of balance to cause an excessive fuel supply, the fuel concentration becomes high throughout the interior of the electrode, and the volume increase also increases. This is particularly remarkable when the power generation is stopped with only the fuel being supplied. On the other hand, if the fuel supply is stopped for an extended period of time after the power generation is stopped, the fuel is lost by crossover, so that the fuel concentration inside the electrode gradually decreases and the volume increase of electrolyte also decreases. Also, when the operation is resumed, the fuel concentration that has lowered during the stop of the power generation needs to be heightened rapidly in order to start power generation in a stable manner.
As described above, due to the unevenness of the fuel concentration inside the electrode and the great change in fuel concentration over time, the electrolyte inside the catalyst layer repeatedly swells and contracts, thereby creating a mechanical stress. The mechanical stress breaks the electrolyte network, so that the electrolyte loses its function as a binder inside the electrode. In the worst case, the electrolyte gets separated with catalyst particles from the electrode and discharged with surplus fuel from the fuel cell. Therefore, the proton conductivity inside the electrode lowers to cause an increase in resistance polarization, or the amount of catalyst decreases to cause an increase in reaction resistance. Consequently, the performance of the fuel cell degrades.